The objectives of this project continue to be the study of the relation between conformational changes in enzymes and the catalytic efficiency of their action. We are studying proteinases from this point of view by using peptide substrates whose structure has been varied systematically, and also have attached groups that can act as fluorescent probes for hydrophobic interaction. Primary attention is being given to pepsin and several related acid proteinases (Rhizopus pepsin, cathepsin D), and this approach is also being applied to the study of papain and chymotrypsin.